1. Field of the Invention
The present invention relates to a tracking servo technique in an optical information recording and/or reproducing apparatus.
2. Description of the Related Art
In an optical information recording and/or reproducing apparatus, a tacking servo is performed so that a light beam used for recording and/or reproducing the information can follow or trace an information track on an optical disc. There are various tracking servo methods. Among them, from a view point of improving a usage efficiency of the light beam emitted from a semiconductor laser, a xe2x80x9cone beam tracking methodxe2x80x9d using only one light beam is advantageous as compared with other methods using a plurality of light beams.
As the one beam tracking method, a so-called (radial) push pull method is well known. The push pull method is a method of detecting a difference between two outputs of two-divided light detector, which is divided in a radial direction of the optical disc (which is referred to as a xe2x80x9cradial directionxe2x80x9d), as a tracking error signal, to thereby detect a drift of the light beam position with respect to the information track on the optical disc.
However, in the above-mentioned push pull method, there is a problem that a drift or shift is generated in a target value of the tracking servo control due to a shift between the objective lens position of an optical pickup and the optical axis of the light detector (which is referred to as a xe2x80x9clens shiftxe2x80x9d hereinafter).
In more detail, the optical pickup has such a structure that an actuator is movably mounted on a slider, and that the objective lens is movably mounted on the actuator. Here, if the disc is eccentric or if the slider does not smoothly move in the radial direction, the actuator performs a fine adjustment of tracking by shifting only the objective lens.
In case that the actuator moves the objective lens in this way, the relative position of the objective lens with respect to the optical axis of the light detector (i.e., the division line of the two-divided light detector) also moves. As a result, the light spot position with respect to the light detector is changed, so that the drift is generated in the target value of the tracking servo.
This phenomenon is explained with referring to FIG. 8. In case that there is no lens shift, the value of the tracking error signal becomes xe2x80x9c0xe2x80x9d under a condition that the light spot is positioned on a central line of the information track. Therefore, the target value of the tracking servo becomes an original point O. However, if there is the lens shift, since the tracking error signal includes an offset component due to the lens shift, the target value of the tracking servo becomes a point Os shown in FIG. 8, so that the drift of the target value is generated. This phenomenon is the more significantly observed as the track pitch of the optical disc is the narrow.
It is therefore an object of the present invention to provide a tracking control apparatus, which can reduce an influence of the lens shift, by using the one beam tracking method.
The above object of the present invention can be achieved by a first tracking control apparatus for a disc having a physical factor changing a shape or a reflection coefficient in a tangential direction of a record track of the disc. The first tracking control apparatus is provided with: a detecting device having a first detector, a second detector disposed adjacent to the first detector in a radial direction of the disc, a third detector disposed adjacent to the second detector in the tangential direction and a fourth detector disposed adjacent to the first detector in the tangential direction and adjacent to the third detector in the radial direction; a first amplifying device for amplifying an output signal of the first detector and an output signal of the fourth detector by a first gain; a second amplifying device for amplifying an output signal of the second detector and an output signal of the third detector by a second gain; a first adding device for calculating a first sum signal, which is a sum of the amplified output signal of the first detector and the amplified output signal of the fourth detector; a second adding device for calculating a second sum signal, which is a sum of the amplified output signal of the second detector and the amplified output signal of the third detector; a first subtracting device for calculating a difference between the first sum signal and the second sum signal and outputting the difference as a tracking error signal; a second subtracting device for calculating a first difference signal, which is a difference between the output signal of the first detector and the output signal of the fourth detector; a third subtracting device for calculating a second difference signal, which is a difference between the output signal of the second detector and the output signal of the third detector; and an amplitude comparing device for comparing amplitudes of the first difference signal and the second difference signal with each other, and determining the first gain and the second gain on the basis of a result of comparison.
According to the first tracking control apparatus of the present invention, the tracking error signal is generated by calculating the difference between the first sum signal, which is the sum of the output signals of the first and fourth detectors, and the second sum signal, which is the sum of the output signals of the second and third detectors. By comparing the amplitudes of the first difference signal, which is the difference between the output signals of the first and fourth detectors, and the second difference signal, which is the difference between the output signals of the second and third detectors, and, on the basis of the result of the comparison, the first gain for the first sum signal and the gain for the second sum signal are determined. Here, since the difference between the amplitude of the first difference signal and the amplitude of the second difference signal indicates the lens shift component, it is possible to obtain the tracking error signal, in which the influence of the lens shift is removed, by adjusting the gains for the first and second sum signals on the basis of this difference indicating the lens shift component.
The above object of the present invention can be also achieved by a second tracking control apparatus for a disc having a physical factor changing a shape or a reflection coefficient in a tangential direction of a record track of the disc. The second tracking control apparatus is provided with: a detecting device having a first detector, a second detector disposed adjacent to the first detector in a radial direction of the disc, a third detector disposed adjacent to the second detector in the tangential direction and a fourth detector disposed adjacent to the first detector in the tangential direction and adjacent to the third detector in the radial direction; a first amplifying device for amplifying a first sum signal, which is a sum of an output signal of the first detector and an output signal of the fourth detector, by a first gain; a second amplifying device for amplifying a second sum signal, which is a sum of an output signal of the second detector and an output signal of the third detector, by a second gain; a first subtracting device for calculating a difference between the first sum signal and the second sum signal and outputting the difference as a tracking error signal; a second subtracting device for calculating a first difference signal, which is a difference between the output signal of the first detector and the output signal of the fourth detector; a third subtracting device for calculating a second difference signal, which is a difference between the output signal of the second detector and the output signal of the third detector; and an amplitude comparing device for comparing amplitudes of the first difference signal and the second difference signal with each other, and determining the first gain and the second gain on the basis of a result of comparison.
According to the second tracking control apparatus of the present invention, the tracking error signal is generated by calculating the difference between the first sum signal, which is the sum of the output signals of the first and fourth detectors, and the second sum signal, which is the sum of the output signals of the second and third detectors. By comparing the amplitudes of the first difference signal, which is the difference between the output signals of the first and fourth detectors, and the second difference signal, which is the difference between the output signals of the second and third detectors, and, on the basis of the result of the comparison, the first gain for the first sum signal and the gain for the second sum signal are determined. Here, since the difference between the amplitude of the first difference signal and the amplitude of the second difference signal indicates the lens shift component, it is possible to obtain the tracking error signal, in which the influence of the lens shift is removed, by adjusting the gains for the first and second sum signals on the basis of this difference indicating the lens shift component.
In one aspect of the first or second tracking control apparatus of the present invention, the amplitude comparing device determines the first gain and the second gain so as to make the amplitude of the first difference signal and the amplitude of the second difference signal equal to each other.
According to this aspect, it is possible to remove the lens shift component in the tracking error signal, by this determining process of the amplitude comparing device.
In another aspect of the first or second tracking control apparatus of the present invention, the apparatus is further provided with: a first holding device for holding the first difference signal and supplying the held first difference signal to the amplitude comparing device; and a second holding device for holding the second difference signal and supplying the held second difference signal to the amplitude comparing device.
According to this aspect, it is possible to continuously monitor the amplitudes of the first and second difference signals.
In another aspect of the first or second tracking control apparatus of the present invention, the apparatus is further provided with a driving device for moving a light beam in the radial direction on the disc on the basis of the tracking error signal.
According to this aspect, it is possible to perform an accurate tracking control, which is not influenced by the lens shift.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.